The invention relates to a method and to a device for image coding of a video signal.
It is primarily intended that the method and device according to the present invention should be utilised in the transmission of video signals that are used, for example, in video telephony. In order to obtain good quality signals for such use, the video signal is converted into a digital signal. The format of the digital signal is specified by the International Consultative Committee for Telegraphs and Telephones (CCITT) H.261 standard. However, the method of digitisation of the video signal can be selected independently by the user.
The transmission of a digital signal is traditionally carried out with the aid of a number of circuits which are frequently divided into a video input signal section, a coder core and a video multiplexer (VMUX). The video input signal section effects suitable signal processing of the video input signal. The coder core effects some form of transformation coding of the signal which can be predictive (INTER) or non predictive (INTRA). After the transformation coding, the signal is quantised.
The digital bit stream is then subjected to further code processing by the VMUX in order to effect compression of the bit stream. Traditionally, run length coding, variable length coding and error correction are also effected. The operation of the VMUX is normally effected by means of a signal processor. Since these codings produce different numbers of bits per frame and the bit rate on the transmission line at the output of the image coding device is constant, the VMUX contains a buffer store.
Problems experienced with the known technology relate to the prevention of overflow in the buffer store and the buffer being emptied prematurely. Both of these problems give rise to a deterioration in image quality. At the same time, the content of the buffer store should be as small as possible.
It is an object of the present invention to overcome the foregoing by arranging for the content of the buffer store to be regulated by adjusting the step height in the quantising arrangements as a function of the difference between an actual and an ideal value for the content of the buffer store.